Since their introduction in 1958 by Can (Carr, 1958), nuclear magnetic resonance imaging (NMR) steady state free precession (SSFP) sequences have, in particular during the last two decades, become increasingly popular for magnetic resonance imaging (MRI) primarily since they combine fast imaging capabilities with high signal-to-noise (Haacke, 1999).
The publications and other materials, including patents, used herein to illustrate the invention and, in particular, to provide additional details respecting the practice are incorporated herein by reference. For convenience, the non-patent publications are referenced in the following text by author and date and are listed in the appended bibliography.
Generally, there are three major groups of SSFP sequences (Scheffler, 1999), RF-spoiled and gradient-spoiled SSFP (SPGR), gradient-spoiled SSFP (non-balanced SSFP: nbSSFP, see FIG. 1a and also U.S. Pat. No. 7,567,081)) and non-RF spoiled non-gradient-spoiled SSFP (balanced SSFP: bSSFP, see FIG. 1b) sequences. In the context of the present invention, the two latter types of SSFP sequences are of particular relevance, namely, nbSSFP and bSSFP. Non-balanced SSFP sequences are generally used for musculoskeletal imaging, since they are insensitive to off-resonances (see FIG. 1a) and offer an excellent fluid to tissue contrast. However, they suffer from motion-related image artifacts. Balanced SSFP sequences are generally used for cardiovascular imaging, since they are motion insensitive. However, they suffer from off-resonance related signal modulations (see FIG. 1b). The frequency response profile (also referred to herein as frequency response function, in particular, its phase and/or amplitude, of bSSFP type sequences has been successfully used for generating functional blood-oxygen-level dependent (BOLD) MRI contrast to assess brain activity (Scheffler et al, 2001; Miller et al, 2003; see also U.S. Pat. No. 7,096,056) and for temperature mapping (Scheffler et al, 2004; see also U.S. Pat. No. 7,078,903). In addition, two methods are known for modulating frequency response profiles of bSSFP which use either alternating repetition times (Leupold et al, 2006) or higher order radio-frequency (RF) phase cycling with identical time of repetition (TR) (Vasanawala et al, 1999; see also U.S. Pat. No. 6,922,054). The particular frequency response functions can be used to map biochemical of biophysical tissue properties within short acquisition times (as achieved with SSFP type of sequences).
However, it is clear that there is a need for additional and/or improved methods and/or apparatuses for generating frequency response profiles that allow mapping functional, biochemical or biophysical tissue information. This invention fulfills, in certain embodiments, one or more of these needs as well as other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.